When completing a well, production zones are provided by submerging a casing string having annular barriers into a borehole or a casing of the well. When the casing string is in the right position in the borehole or in another casing in the borehole, the annular barriers are expanded or inflated. The annular barriers are in some completions expanded by pressurised fluid, which requires a certain amount of additional energy. In other completions, a compound inside the annular barrier is heated so that the compound becomes gaseous, hence increasing its volume and thus expanding the expandable sleeve.
In order to seal off a zone between a well tubular structure and the borehole or an inner and an outer tubular structure, a second annular barrier is used. The first annular barrier is expanded on one side of the zone to be sealed off, and the second annular barrier is expanded on the other side of that zone, and in this way, the zone is sealed off.
When expanded, annular barriers may be subjected to a continuous pressure or a periodically high pressure from the outside, either in the form of hydraulic pressure within the well environment or in the form of formation pressure. In some circumstances, such pressures may cause the annular barrier to collapse, which may have severe consequences for the area which is to be sealed off by the barrier, as the sealing properties are lost due to the collapse. A similar problem may arise when the expandable sleeve is expanded by an expansion means, e.g. pressurised fluid. If the fluid leaks from the sleeve, the back pressure may fade, and the sleeve itself may thereby collapse.
The ability of the expanded sleeve of an annular barrier to withstand the collapse pressure is thus affected by many variables, such as strength of material, wall thickness, surface area exposed to the collapse pressure, temperature, well fluids, etc.
A collapse rating currently achievable for the expanded sleeve within certain well environments is insufficient for all well applications. Thus, it is desirable to increase the collapse rating to enable annular barriers to be used in all wells, specifically in wells with a high drawdown pressure during production and depletion. The collapse rating may be increased by increasing the wall thickness or the strength of the material, however, this would increase the expansion pressure, which, as already mentioned, is not desirable.